Well, that's a nice question you got right there. And thank you for asking.
I read somewhere a long time ago that everybody thinks they can write one book. I'm not sure it is true, but I'm pretty sure most people can come up with one book.
It does not have to be a work of fiction. But everything knows something about a subject, has a fiction story on the ready, or had a life worth narrating. Well for the last one, you have to wait until you're at least 40.
There is no telling it would sell or even get published though (self-published work does not count).
So what about me ?
Well I've been reading books since i could read basically. Based on what I gathered from my years in psychology, I'll say the main reason for that was an educated father, always reading history, works of fiction and so on.
And after reading stories in books, i made up stories for my toys. Because after all Lego men are not just pieces of plastic, they where heroes of their own world.
Then i would just tell myself stories when i got bored. I long train/subway/car trips, i would invent worlds and watch/have adventures in it. Epic adventures, i might add.
Oral presentations at school were also my forte, because you could turn any subject into a story : review the protagonists (be they historic figures or atoms...), tell where they came from, where they went, and what happened when they crossed paths.
Then a few years back i got my first gig as a writer. Granted, i was not mystery or even fiction writer : i wrote (and still do) articles about security forces, weapons, tanks, and all that. But it got me going.
A few years after that I decided i would write a tentative first book. That did not go far. Well i'm telling that but it got to the point where i have a complete story with every twist detailed (pretty much a movie storyboard, without the pictures, they are in my head). In two years i wrote about 10 short chapters.
Then i got my first book contract. Once again, not for anything fancy, just for an 84-pages book about the US Army in 2012.
But it got the machine running again.
So now i have four books in my metaphorical drawers. One is my old project, going back to 2009 and not going anywhere fast. The second is a WWII story with a twist, with so much research needed i don't have the physical time or money to get it rolling (or maybe it's just me finding an excuse to do nothing).
The third one is a brand new story using the characters and settings of my first project (i'm stripping it for parts but the frame is left intact for completion and/or re-writing later) and is running not as full speed, but at a nice steady pace (3 weeks have produced 23 chapters for about 50/60 pages at 1500 sign/page) so far.
the last one came at me one night and has been shaping up nicely, but will demand a more dense approach, being set up in a parallel timeline (the 3 other project use the events of our very own timeline for the time being).
And fear not, all of those will feature blackjack and/or hookers.